1. Field of the Invention
The present invention relates to an automatic flash control device for a photographic operation using an electronic flash in a camera having a focal plane shutter and, more particularly, to a TTL multiple flash control device based on a concept of multiple pattern photometry.
2. Related Background Art
A conventional camera having a TTL automatic flash control function includes one light-receiving element for performing a photometric operation of the entire film surface or a major portion thereof.
In operation of this camera, after a shutter is fully opened, an electronic flash emits light, and object image light reflected by the film surface is photoelectrically converted by a light-receiving element. A signal corresponding to an integrated value of the quantity of light of the light-receiving element is compared with a predetermined value. The amount of light from the electronic flash is controlled so that the film surface has a predetermined brightness level.
This scheme has a disadvantage in that various conditions in the field of view cannot be sufficiently satisfied because a detection area of the field of view is one-dimensional. As a result, an optimal exposure value of the principal object cannot be obtained.
In recent years, techniques for solving the above problem are being developed.
As one of these techniques, Japanese Patent Application No. 1-203735 proposes the following system by employing the concept of so-called multiple pattern photometry in TTL automatic flash control.
A plurality of photoelectric converting means capable of performing photometry of a plurality of areas constituting the field of view are arranged at a position where the quantity of light on the film surface is measured, and an electronic flash is caused to preliminarily emit light immediately before a focal plane shutter is opened. The light from the electronic flash is reflected by the object image, and the reflected light is reflected in turn by the surface of a shutter curtain. The resultant light is received by the plurality of photoelectric converting means, and outputs from these photoelectric converting means are independently integrated. The integrated values are detected as field reflection distributions of the areas upon flashing.
The detected field reflection distribution information of each area is arithmetically operated in accordance with a predetermined multiple pattern algorithm. Therefore, the degrees of weighting of the respective divided areas are determined to optimize exposure for the principal object.
As soon as the shutter is opened, the electronic flash emits light, and light reflected by the film surface is received by the same plurality of photoelectric converting means. After the outputs from these photoelectric converting means are weighted with predetermined weighting coefficients, the weighted outputs are added and integrated. The resultant value is compared with the predetermined value to stop flashing of the electronic flash at a timing determined by this comparison, thereby completing automatic flash control.
This TTL automatic flash control scheme is called "TTL multiple flash control".
This TTL multiple flash control is characterized in that the plurality of photoelectric converting means are located at a position where light reflected by the shutter surface capable of performing photometry in each of the plurality of divided areas constituting the field is received, and that each of independent integrated values of the values output from the plurality of photoelectric converting means is defined as field reflection distribution information of each area, and this information is arithmetically operated in accordance with the predetermined multiple pattern algorithm.
When an integrated voltage as an output from each -photoelectric converting means is to be A/D-converted and digital data is to be input to a microcomputer, the quantity of light is decreased with respect to the quantity of light corresponding to a full A/D conversion scale such that a quantity of light corresponding to a one-stopped-down value (1 EV) is 1/2 the full scale, a quantity of light corresponding to a two-stopped-down value is 1/4 the full scale, a quantity of light corresponding to a three-stopped-down value is 1/8 the full scale, and the like. Therefore, the A/D conversion resolution is undesirably decreased.
For example, when an 8-bit A/D converter is used, with a decrease in light quantity, a photoelectric conversion value allowing data read at a resolution of about 8 LSB/stop is a maximum of a 5-stopped-down value of the quantity of light corresponding to the full scale.